In the following description the term “turbine” is used to refer to rotary engines having a stator and a rotating part force coupled by a fluid medium such as steam or gas. Of particular interest for the present invention are axial turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades. Movements are generally registered as movements relative to a casing or housing.
The moving blades are designed to have a root at the bottom end to assemble with rotor and a shroud at the top end to engage with the shrouds of adjacent blades forming a ring. The moving blades are assembled on to the rotor having grooves in axial or circumferential direction. The axial grooves can be straight or curved.
It is known to assemble a row of blades such that each blade is twisted. The process of twisting can be regarded as forcing the blade from its unconstrained equilibrium state into a twisted state by applying mechanical constraints, typically by the forces applied at the root section and the top or shroud section.
A particular problem sought to be avoided is the excitation of natural or eigen frequencies of the blade (s) in turbine designs. Any type of resonant behavior of the blade or blade assembly has potentially a harmful impact upon the operation of a turbine and is hence to be avoided. A way of avoiding the resonances during operation is seen by using stiffer blade profiles and/or increased twist angles. Both solutions have disadvantages leading to increased stresses or difficulties in assembling a row of blades with a high degree of pre-stress. Blades assembled with higher pre-twist can be prone to bending instead of twisting as desired. It is therefore seen as an object of the invention to improve existing blade designs to increase the frequencies of the blade while at least partially avoiding the problems associated with previous solutions.